The present invention relates to an electrical machine, in particular a universal motor.
Universal motors are used widely, e.g, in power tools. Since, with universal motors, electric current flows through the armature winding and the excitation winding, a great deal of heat is produced, which gradually heats up the entire motor. To prevent damage that could be caused by the motor heating up, it must be ensured that the motor is cooled adequately. With power tools, air is drawn in from the outside, e.g., through a fan wheel, for cooling, as described, e.g., in DE 102 56 805 A. The fan wheel is mounted non-rotatably on the armature shaft between the armature and gearbox. When the fan wheel rotates, it generates an air flow which flows axially through the power tool and carries heat to the outside.
To achieve good cooling, as much as air as possible must be transported through the power tool. High air throughput means a high flow rate, however, which can be associated with undesired fan noises under certain circumstances. In addition, when the cooling air flows through the power tool, it only passes over the surface of the armature and the stator. With universal motors with a small armature diameter in particular, the surface of the armature and the stator is correspondingly small, so that a relatively small amount of heat can be given off to the cooling air by the armature and the stator. Cooling using a fan wheel is therefore inadequate.